The relation between the stimulus (subjective) and reinforcing (rewarding) effects od drugs is largely unknown. At one extreme it is possible that these effects are unrelated. In contrast, it could be that the stimulus effects of drugs are the reinforcing effects. Recent evidence suggests that the stimulus and reinforcing effects are linked; self-administering a drug has been shown to increase sensitivity to its stimulus effects. The present project investigates this phenomenon and also addresses the reverse question of whether the degree to which monkeys self-administer drugs can be altered through drug discrimination manipulations that increase or decrease sensitivity to the stimulus effects of the drug. For example, the degree to which squirrel monkeys self-administer ketamine (a phencyclidine-like drug) will be assessed before and after they have been trained to discriminate a low dose of ketamine from saline. If the stimulus effects of a drug are related to its abuse potential then it would be expected that this manipulation would increase the consumption of low doses of ketamine. Conversely, training monkeys to discriminate low doses of ketamine from high doses could decrease the extent to which low doses are self-administered. In addition, some drugs are more likely to be self-administered in monkeys who have previously self-administered other drugs with similar stimulus effects. This is important, for it may be related to the notion that some drug experiences may be "gateways" to abusing other drugs. This issue will be examined by training monkeys to categorize pairs of drugs as similar to or different from one another and examining the impact of these manipulations on their co-morbidity of abuse. Understanding the relation between the stimulus and reinforcing effects of drugs will contribute considerably to the overall understanding of the determinants of drug abuse by providing evidence that the stimulus effects of drugs are related to their potential for abuse. This line of inquiry could eventually lead to the development of new discrimination- based clinical interventions designed to reduce the extent to which humans self-administer drugs of abuse.